A gas tanker truck explosion along a highway in Mexico City killed three people and injured 67 more, the mayor of Mexico City said on Wednesday. 

The vehicle blew up under a highway overpass in the populous Iztapalapa district in the southeast of Mexico's capital.

The explosion sent flames and smoke billowing over the area, causing widespread damage and burning numerous other vehicles.

Nineteen people are seriously wounded with second- and third-degree burns, local officials said, adding that three people died of their injuries. 

Calling the explosion an "emergency," Mexico City's head of government, Clara Brugada, said the wounded had been taken to hospitals across the city.

Brugada said prosecutors were investigating the incident, but it appeared that the truck exploded after it tipped over on the highway.

Images showed the burning truck, which was transporting 49,500 liters (13,076 gallons) of gas, overturned on the road.

The explosion occurred on one of the most important roadways flowing out of Mexico City on the way to the city of Puebla.

The city's government said the roadway would be closed until further notice.